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As these platforms collapse, they endanger the continent's glaciers and set the stage for further sea level rise.
Antarctica's ice shelves have lost nearly 4 trillion metric tons of ice since the mid-1990s, scientists say. Ocean water is melting them from the bottom up, causing them to lose mass faster than they can refreeze.
That's according to a new study looking at satellite data from 1994 to 2018. The results were published yesterday in the journal Nature Geoscience .
That is bad news for the hundreds of glaciers that stretch along the Antarctic coastline.
Ice shelves are ledges of ice that jut out from the edge of the continent into the ocean. They help keep glaciers stable, keeping them in place.
As ice shelves melt, they become thinner, weaker and more likely to break. When this happens, they can release streams of ice from the glaciers behind them, raising global sea levels.
Scientists have become more concerned about the Antarctic ice shelves in recent years. Research increasingly suggests that ice shelves in certain regions of the continent, particularly West Antarctica and parts of the Antarctic Peninsula, are melting and thinning from the bottom up.
The new study confirms that the fastest-melting areas are primarily ice shelves jutting into the Amundsen and Bellingshausen Seas, stretching back to the coast of West Antarctica and the western flank of the Antarctic Peninsula.
Melting ice shelves represent only part of the ice that Antarctica loses in a given year.
Research suggests that the continent is losing billions of tons of ice every year. Part of the mass loss comes from melting of ice sheets and part from melting at the surface of the ice sheet. Most of it comes from chunks of ice that pour from glaciers into the sea.
And the thinning and weakening of the ice shelves can accelerate that process.
Warm ocean water currents appear to be to blame. This warm water originates from the Pacific and Indian Oceans and flows south into Antarctica.
It is usually a deep water current. But when it reaches the Antarctic continent, some of it can bubble up to the surface. There, it can seep under nearby ice shelves and melt the ice from the bottom up.
The new study shows that the fusion has not been totally constant over time. The melting appeared to accelerate in the late 2000s, before finally slowing down again in the 2010s.
That's likely due in part to the influence of El Niño and La Niña patterns in the Pacific Ocean, according to lead study author Susheel Adusumilli, a doctoral student at the Scripps Institute for Oceanography at the University of California, San Diego. . These natural weather patterns can cause ocean temperatures to oscillate between warmer and colder cycles.
Still, many researchers believe that climate change is also likely contributing to the melting of ice shelves. Research suggests that climate change may influence certain wind patterns around Antarctica, which can churn the waters of the Southern Ocean and increase the amount of warm water that rises to the surface.
Modeling studies suggest that this process may become more intense in the coming decades as the Earth continues to warm.
And even when melt rates are slower than in the past, ice shelves continue to lose mass overall.
If the ice shelves were in a stable state, then they could swing between gaining mass and losing mass, Adusumilli noted.
But for the past 25 years, "there is always a massive loss," he said. “It goes from a small amount of mass loss to a large amount of mass loss to a small amount of mass loss again. It never goes from a mass gain to a mass loss ”.
Sea level rise is the biggest concern for Antarctic glacier ice loss. But the melting of ice shelves is worth keeping an eye on for another reason, Adusumilli added.
The influx of meltwater into the sea can alter the ocean in significant ways.
Cold, fresh water can form a rigid layer on the ocean's surface. Some researchers believe that this, in turn, could allow the deeper and warmer layers of the ocean to get even warmer.
And when these warm layers build up on the edge of the continent, they could cause the ice shelves to melt even faster.
"People talk about how further melting of the ice shelf can lead to more ice discharge on land and rising sea levels," Adusumilli said. "But the immediate influence of the ice shelves in the ocean is also very important."